“Piston, golf-type,” said Yvon Duhamel’s mechanic, Steve Whitelock, holding up a two-stroke piston just like this one. “Hole-in-one.”

This is a classic case of pre-ignition, which is ignition of the fresh charge long before the timed ignition spark. As this piston came up on compression, it was trying to compress flaming high-pressure combustion gas.

Why didn’t the engine just stop? It didn’t because it had three cylinders; the power of the other two cylinders wouldn’t let it stop.

People used to believe wonderful nonsense about pre-ignition, such as the idea that it was caused by “the spark being too near the piston crown.” More than one manufacturer tried tall “witch-hat” combustion chambers that moved the sparkplug far from the piston dome. No improvement.

In recent times, instrumentation has made it possible to see exactly what happens in pre-ignition. The usual cause—as it certainly was in the case of this piston—is a sparkplug of a heat range too hot for the engine. Ideally, the sparkplug electrodes and the tip of the plug’s central insulator should run hot enough to prevent plug-fouling carbon from condensing on them but not hot enough for any of those parts to act as an ignition source.

When a too-hot plug electrode or a bit of carbon deposit glows red and acts as an ignition source, the charge usually lights up as it enters the cylinder—somewhere near bottom center. That gives the flaming gases plenty of time to heat the piston crown. The part of the crown farthest from the cool cylinder wall—the very center—heats up the fastest. It takes only a very few crank revolutions for this part of the piston dome to soften enough for the very high pressure to blow it through.

Notice that the piston crown is clean—almost no carbon deposit. This was a new piston. As it got hot from the pre-ignition, it also began to seize; you see the bright vertical smears made by melting of the piston as it expanded hard against the cylinder wall, hot enough to boil any lubricant.

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